I bought a ring, returned it, & I'm being charged ( account in collection)

I bought a ring in Jewelry store, returned it, owner gave me a credit slip, stating ring was returned, and now I'm in accounts collection...This has been going on for 2 years..A financial counselor at my Senior Center wrote to Calvary ( Synchrony Bank) and called them, and I thought the problem was solved and now a year later I get another letter ...I went to Bar Association, they suggested I write them a certified letter..I did, and they're saying I have to pay this debt of 4078.10....I RETURNED THE RING, so why do I have to pay for something I returned...... I am 72 year olds, with health problems, on a limited income......Aside from this, I have always had excellent credit...and this is upsetting...